Photodynamic therapy (PDT) involves the use of light of an appropriate wavelength to excite an administered photosensitizer resulting in tissue destruction. A purified form of hematoporphyrin derivative (HPD) , high in dihematoporphyrin ethers/esters, (DHE) , is the photosensitizer most frequently used in PDT. HPD and DHE are selectively retained in tumors allowing for tumor destruction while sparing normal structures during PDT. This process is distinct from psoralen-based light therapy commonly used in dermatology. Cutaneous disorders are particularly well suited to PDT because their accessibility allows for adequate light administration. Basal cell carcinoma, squamous cell carcinoma, melanoma, mycosis fungoides, Kaposi's sarcoma, metastatic breast carcinoma, and Bowen's disease all have been treated with PDT using HPD or DHE.
Photodynamic effect is a function of 4 variables: the light dose given, tissue DHE content, melanin and other substances competing with DHE for light absorption, and tissue oxygen content. Light delivery is usually accomplished with an argon or argon-pumped dye laser and may be closely controlled for total light dose and treatment area. Since PDT requires oxygen to exert a cytocidal effect, hypoxic tissues are relatively resistant to treatment with PDT.
Tissue DHE content is a function of the administered dose of DHE and retention within various tissues. Different tissues and tumors retain varying amounts of DHE. Individual variations in retention and excretion of DHE may result in differences in DHE content in similar individuals. Currently, tissue and tumor DHE are estimated based on the amount of DHE administered. By overestimating tumor DHE content, inadequate light may be administered which subsequently results in decreased tumoricidal effect. An underestimation of DHE present may result in over treatment of normal structures with the result being unacceptable normal tissue destruction. In addition, measurements of target and normal tissue DHE may be made repeatedly over time as DHE is cleared. Thus, light delivery should be timed to coincide with maximal target tissue retention of DHE over normal structures. A quicker method of measuring relative tissue DHE content would allow for more accurate light dosing, optimizing PDT.
The present invention provides for a non-invasive method and apparatus for accurately measuring the relative amount of tissue DHE content.